This invention relates to packaging materials useful for microwave cooking applications, and particularly to packaging materials which will brown and crispen food items without overheating and degradation of the seams of such packages.
There has been much interest recently in packaging materials which aid in browning and crispening of food items in a microwave oven. U.S. Pat. No. 4,267,420, to Brastad, discloses a food item wrapped with plastic film having a very thin coating thereon. The film conforms to a substantial portion of the food item. The coating converts some of the microwave energy into heat which is transmitted directly to the surface portion of the food so that a browning and/or crispening is achieved.
U.S. Pat. No. 4,641,005, Seiferth, discloses a disposable food receptacle for use in microwave cooking, which includes a provision to brown the exterior of the food in the receptacle. A thin layer of an electrically conductive material is incorporated into the receptacle on the food contacting surfaces thereof, so that the conductive layer will become heated by the microwave radiation and will, in turn, brown the exterior of the food in the receptacle. The receptacle includes a smooth surfaced plastic film, as a protective layer, and a support means formed of paper stock material.
U.S. Pat. No. 4,713,510, Quick et al., discloses a microwave ovenable package including a layer of material that will convert a portion of the microwave energy to heat and a layer of paperboard interposed between the energy-converting layer and the food. The energy-converting layer may be carried on a plastic film, and an additional layer of paperboard may be used to sandwich the energy-converting layer and the plastic film between layers of paperboard. For the purpose of providing a more intense heating effect, two energy-converting layers, each on a dielectric substrate, sandwiched together between layers of paperboard, are disclosed.
U.S Pat. No. 4,735,513, Watkins et al., discloses a flexible sheet structure comprising a base sheet having a microwave coupling layer which may be in the form of an island covering a selected area of the sheet. The sheet may be laminated to a backing sheet of dimensionally stable flexible material transparent to microwaves; backing sheets can be applied to both sides of the base sheet. The structure may have unheated portions which are adapted to be folded, tucked, and wrapped around a product to be heated.
Copending U.S. Pat. Application No. 188,556 discloses a conformable laminated wrap for packaging articles of food requiring browning and crispening and a degree of shielding during microwave cooking. The laminated wrap has at least two layers of heat resistant microwave transparent plastic film, and at least one substantially continuous layer of microwave susceptive material, which is coated on at least one of the interior surfaces or interfaces formed between the plastic films of the laminate.
The use of shields to reduce the amount of microwave radiation reaching food items is also known in the art. For example, U.S. Pat. No. 4,190,757, Turpin et al., discloses the use of metal foil to totally shield food from microwave energy except that which enters through an opening in the shield. The foil may be bonded to a layer of microwave lossy absorptive heating composition, as seen e.g. in FIG. 7 of that patent.
In packages suitable for browning or crispening food in a microwave oven, seals or seams may be present and microwave susceptive heating materials may be present near or as a part of the seal. Alternatively, areas of a microwave package coated with a microwave susceptor may be located in contact with each other, yet not in close contact with food. A problem is often encountered with overheating and in severe cases even melting of the film in such areas where there is no close contact with food to act as a heat sink. Melting of the polymer can cause the package to deform and the layers of film to adhere together, and in severe cases may even cause contamination of the food with molten polymer. The present invention avoids these problems by selectively providing shielding for such areas, thereby providing a package in which the film remains unmelted and the package remains easy to open.